This invention relates to a process for electroless copper plating which can form a copper plating film having distinguished mechanical properties, and particularly to a process for electroless copper plating suitable for thick plating in the production of printed circuit boards.
However, a process for electroless copper plating to deposit metallic copper by adding a copper ion-reducing agent such as formaldehyde, etc. to a solution containing copper ions and its complexing agent has been known. However, the electroless copper plating solution consisting only of such basic components is unstable and liable to undergo self-decomposition reaction. Thus, it has been difficult to carry out the electroless copper plating for a long time.
Japanese Patent Publication No. 43-12966 discloses addition of a sulfur compound such as thiourea, 2-mercaptobenzothiazole, etc. thereto to improve the stability of the plating solution. However, even if the resulting plating solution containing the additive sulfur compound is applied to a process for producing printed circuit boards according to the so called additive process which forms a conductor circuit for the printed circuit board or plating throughholes by electroless copper plating, the mechanical properties of the resulting plating films are not satisfactory, and novel additives to an electroless copper plating solution which can form a plating film having better mechanical properties have been desired.
Furthermore, an electroless copper plating solution capable of producing a plating film of distinguished mechanical properties is necessary for printed circuit boards for throughhole interconnections by thick plating, and an electroless copper plating solutions containing a further additive such as a cyano compound, 2,2'-dipyridyl, polyethylene glycol, etc. besides said basic components are known. However, these additives have problems in handling and are consumed during the plating. Thus, their analysis and make-up are necessary from time to time during the plating.
Furthermore, a process for improving mechanical properties of a copper plating film by electroless copper plating with an electroless copper plating solution containing said basic components 2,2'-dipyridyl, polyethylene glycol and an alkali-soluble inorganic silicon compound is disclosed [Japanese Patent Application Kokai (Laid-open) No. 54-19430], where it is stated that the resulting electroless copper plating film having a tensile strength of 50 to 58 kg f/mm.sup.2 and an elongation of 4.4 to 6.7% can be obtained when 5-100 mg/l of the alkali-soluble inorganic silicon compound in terms of silicon dioxide (0.08 to 1.7 m moles/l in terms of Si) is contained in the electroless copper plating solution. However, the stability of the plating solution is not good and there takes place abnormal deposition (a phenomenon of depositing copper on outside of desired portions) when the plating solution is used continuously for a little prolonged time.
It has been so far known to add an inorganic silicon compound such as water glass, sodium metasilicate, and sodium orthosilicate to the electroless copper plating solution, but the resulting electroless copper plating solution is not satisfactory in the stability. For example, the plating solution becomes unstable in the course of plating when applied to thick electroless copper plating, and a plating film having a satisfactory thickness cannot be substantially obtained. This is obvious from the disclosure in "Kinzoku Hyomen-Gizitsu (Metal Surface Technology) Vol. 16, November issue (1965) that the inorganic silicate compound is effective for preventing the self-decomposition of the electroless copper plating solution, but copper deposits at the bottom of the plating tank. Furthermore, the plating film obtained from the electroless copper plating solution as described in said reference is brittle and has no satisfactory mechanical characteristics upon measurement of the physical properties of the plating film. That is, as shown in Tables 5 and 6 which follow, it has been found that mere addition of an inorganic silicon compound to the electroless copper plating solution causes copper deposition in the plating tank. Furthermore, it has been found that the plating solution undergoes self-decomposition during the plating, depending on the species of complexing agents, and the mechanical properties of the plating film obtained from such a plating solution are not better in general.
On the other hand, it is disclosed in Japanese Patent Publication No. 36-9063 to inject an oxygen-containing gas, for example, air, into an electroless copper plating solution with an effect on the stabilization of the plating solution. However, mere injection of oxygen-containing gas cannot remarkable improve the mechanical properties of the plating film. That is, as shown in Tables 5 and 6 which follow, it has been found that the self-decomposition of the plating solution can be suppressed and the stability of the plating solution can be improved to some degree by injection of air into the plating solution, but the mechanical properties of the plating films obtained by mere injection of air into the plating solution are not improved. That is, mere injection of oxygen-containing gas cannot produce satisfactorily distinguished mechanical properties.